This invention relates to apparatus for providing a reference laser beam and, more particularly, to a laser beam transmitter having an improved lighthouse structure. The lighthouse of the present invention effectively seals the transmitter against moisture and dirt. Further, this lighthouse is rugged, while also facilitating repair should a glass panel be broken during use of the transmitter.
Laser beam systems have been employed in numerous surveying and construction applications. In one such system, disclosed in U.S. Pat. No. 4,062,634, issued to Rando et al, a laser beam transmitter produces a rotating laser beam which establishes a reference plane. The reference plane swept by the rotating beam is used as an elevational reference over an entire work area, such as for example a construction site. Also, the system employs one or more laser beam detectors, placed at considerable distances from the transmitter, for intercepting the rotating laser beam in the reference plane. These detectors may be carried on earthmoving equipment and used by the operators of the equipment to determine proper elevation at various points throughout the work area.
In a laser beam transmitter of this type, the horizontal rotating reference laser beam is produced by projecting the beam generally upward, and then redirecting it ninety degrees within a pentaprism assembly. The pentaprism assembly, which typically uses a pair of mirrors rather than a prism to produce the needed double reflection, is rotated about a vertical axis within the projecting apparatus to cause the resulting horizontal beam to rotate, thereby defining the reference plane. If desired, the beam may be rotated in a plane that is tilted by a specified amount with respect to horizontal, thereby providing a tilted reference plane.
To adapt the reference laser beam transmitter for use in surveying and construction applications under a variety of field conditions, the transmitter is provided with a waterproof housing having an upper, transparent lighthouse assembly within which the pentaprism assembly rotates and through which the laser beam is projected. In the transmitter shown in the '634 patent, the lighthouse assembly includes four upstanding flat transparent panes of glass of high optical quality which are joined together at their side edge surfaces so as to form a square in cross section. The lower edges of the panes are mounted within recessed seats formed in an upper rim of the housing, while the upper edges of the panes are mounted within recessed seats formed within a top casing of the housing. Gaskets seal the upper and lower edges of the panes. Four posts extend between the top casing of the housing and upper rim of the housing and apply sufficient compressive force to pull them together and lock the panes into their seats. The four posts are aligned radically outward from the corners of the lighthouse assembly. Where the panes meet at the corners they have forty-five degree beveled side edges that mate and are further sealed by a suitable adhesive, such as a silicone adhesive.
While this lighthouse construction performs well, it requires careful grinding of the individual glass panes to precise dimensions to bevel the side edge surfaces that form the corner junctures of the lighthouse. The assembly of the lighthouse also requires precise positioning of the panes relative to one another, before they were adhered together. Additionally, this lighthouse construction is not as rugged as might be desired. Several lighthouse constructions have been used to overcome these difficulties. One such construction, similar to that in the '634 patent, is shown in U.S. Pat. No. 4,776,672, issued Oct. 11, 1988. In the lighthouse to which the '672 patent is directed, a plurality of transparent panes are arranged in a generally polygonal cross-sectional shape. The window structure has spaced corners formed by pairs of adjacent ones of the opposite side edge portions of the panes. The side edge portions of the panes in each pair are offset relative to one another in forming a respective corner. The offset position of the side edge portions of each pair in forming a respective corner exposes only an interior edge on one of the adjacent side edge portions to laser light being transmitted through the window structure. The path of laser light through the adjacent side edge portions forming the corner is thereby affected only by one interior edge on the respective one of the adjacent side edge portions of the panels at each corner. This reduces the care with which the edges of the panes must be adhesively bonded, but does not deal with the issue of ruggedness.
Another lighthouse construction is shown in U.S. Pat. No. 4,662,707, issued May 5, 1987. The '707 patent discloses a lighthouse configured as a cylindrical lighthouse of molded acrylic resin. The lighthouse has no corners and, therefore, no problems of the type associated with joining together flat panes at the corners. A difficulty with the cylindrical configuration, however, is that the rays of laser light at the periphery of the beam will tend to diverge laterally from the center of the beam after being refracted twice as they pass through the lighthouse. To overcome this difficulty, an additional correction lens is provided in the '707 transmitter, to minimize the generally horizontal beam divergence. While this effectively eliminates several limitations encountered with respect to prior art lighthouses, it also requires the addition of a component, the lens, and the resulting inaccuracies.
Accordingly, it is seen that there is a need for an improved lighthouse construction in which additional optical components are not required. Further, there is a need for a lighthouse construction in which the lighthouse is rugged, and easily constructed.